SCC Susceptibility Analysis Of Low-alloy High-strength Steel Pressure Vessels In Wet H₂S And Nitrate Environment And Study Of Safety Control Technology

The stress corrosion cracking (SCC) under the action of fluid environment is one of the salient problems that influence the safety of pressure vessels. This work is supported by the special fund for the important technical problem tackling subject under the national 9th five-year plan (96-918-02-04) and special fund for social and public welfare research subject of the Ministry of Science and Technology (043GS). Based on the experimental study, failure mechanism and analysis of practical engineering cases, the generation mechanism and susceptibility conditions, which are about the SCC of series of steel 07MnCrMoVR, 15MnNbR steel and 16MnR steel for pressure vessels and the welded joints of them in wet H2S, were systematically analyzed. According to the analysis, the engineering estimation expressions for the mechanical parameter KISCC and dynamic parameter da/dt that measure the SCC conditions of typical steel grades under the critical condition are derived, then the essence for the cracking of typical pressure vessels is revealed and the engineering method for preventing the SCC in wet K₂S is proposed. Moreover, by simulating the fluid condition of regenerators, SCC experiment in nitrate environment was also conducted to study the cracking phenomenon of the catalytic regenerators of petrochemical plant, and the engineering method for safety control is presented. The main contents of this paper are as follows:(1) SSRT test, Kiscc and da/dt test using modified WOL specimens and immersion test for hydrogen-induced cracking (HIC) resistance are relatively systematically carried out for 07MnCrMoVR series steel, 15MnNbR steel and 16MnR steel and their welded joints under wet H2S environment, the laws of effects of H2S concentration and PH value in the medium, welded joint and heat treatment condition, purity of steel, surface coating, etc. on macroscopic susceptibility to SCC are analyzed in the tests, the lower limit of Kiscc and range of plateau crack growth rate da/dt of the three types of steel and their welded joints are measured, and the behavior and influential factors of the three types of steel resistance to HIC and hydrogen burble (HB) are experimentally analyzed. Among them, systematic measurement of the Kiscc and da/dt of 07MnCrMoVR and 15MnNbR steel considering effect of heat treatment condition of welded joints is the first time in China.(2) From hydrogen origination, diffusion and concentration, hydrogen embrittlement cracking mechanism of pressure vessel steel in wet H2S environment is analyzed in detail. Considering the difference between the two major failure modes of teSrSpecific low-alloy steel and in combination with fracture mechanics model, the estimation expressions for critical threshold values Cth of the three types of steel under HIC and HB environment and expression for Kiscc under sulfide stress corrosion cracking (SSCC) condition are derived respectively, and have better correspondence to test results, which can be taken as engineering estimation method.(3) Based on the Gerberich model, the expressions of da/dt for the three stages of the SCCin which the dominant course of control is the diffusion are derived. According to the expressions, the formulas to estimate da/dt of plateau for the three types of steel that are 16MnR, 15MnNbR and 07MncrMovR steel are given. It is found that the theoretical values are in good agreement with the test results under the high-density state of HaS. So the present formulas can be taken as the approximate engineering estimation method for HbS of high concentration.(4) According to the cracking mechanism and experimental study, the factors that lead to cracking in wet HbS are discussed. The concept of the so-called "limit concentration" at which SSCC never takes place is discussed in detail. From the discussion a new viewpoint is proposed that so long as there is stress gradient great enough and sufficiently long time, the SCC of HbS will still arise even the 828 density is smaller. This viewpoint has been verified in th...